The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for improving the performance of energized cement systems for well cementing applications.
Primary cementing is the process of placing cement in the annulus between the casing and the formations exposed to wellbore. Alternatively, cement may be placed in the annular region between two casing strings. The major objective of primary cementing is to provide zonal isolation in wells; for example, to exclude fluids such as water or gas in one zone from oil in another zone in the well, or to protect aquifers from contamination by oil or gas emanating from zones further downhole. To achieve this objective, a hydraulic seal is created between the casing and cement and between the cement and the formations, while at the same time preventing the formation of fluid channels within the cement sheath.
The basic process for accomplishing a primary cementing job employs the two-plug method for pumping and displacement. After the well is drilled to the desired depth, the drillpipe is removed and a larger-diameter casing string is normally run to the bottom of the well. At this time, drilling mud is still in the wellbore. This mud is then displaced, removed from the well and replaced by a cement slurry. To prevent contamination by mud, two plugs isolate the cement slurry as it is pumped down the casing. Sufficient cement slurry is pumped into the casing to fill the annular space from the bottom to at least a level that covers all of the productive zones. In addition to providing zonal isolation, the cement sheath in the annulus protects the casing from corrosion. After slurry placement, the well is shut in for a time sufficient to allow the cement to harden before completion work begins or drilling commences to a deeper horizon.
Remedial cementing consists of two broad categories: squeeze cementing and plug cementing. Squeeze cementing is the process of placing a cement slurry into a wellbore under sufficient hydraulic pressure to partially dehydrate or expel water from the cement slurry, leaving a competent cement that will harden and seal voids. Plug cementing is the placement of a limited volume of cement at a specific location inside the wellbore to create a solid seal or plug. Remedial cementing operations are performed for various reasons: to repair faulty primary cementing jobs, alter formation characteristics, repair casing and abandon wells.
For primary or remedial cementing to be successful, the cement should bond intimately with the surfaces to which it is attached—casing or formation rock. The industry has developed numerous techniques for achieving good bonding. For example, measures are usually taken to ensure that drilling fluid is efficiently removed from the annulus, ensuring that the bonding surfaces are clean and water-wet. Latexes may be added to the cement slurry to improve adhesion of the cement to the casing and formation. Or materials may be added to the cement slurry that cause the set cement to expand after it sets.
Another method for achieving improved bonding is to “energize” the cement slurry by generating gas inside the slurry in situ during placement and the setting phase. Such slurries are compressible and the resulting pressurization may ensure optimal contact between the slurry and the bonding surfaces. The presence of gas in a cement slurry may also improve fluid-loss control and help prevent migration of formation fluids into the annulus before the cement sets.
A thorough description of the primary and remedial cementing, as well as methods for improving bonding, may be found in the following publication. Nelson E B and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006).